1. Field of the Invention
The invention relates to a hydraulic pressure control apparatus for a vehicular power transmitting device which is provided with a direct-coupled clutch control solenoid valve that controls operation of a control valve for switching a direct-coupled clutch provided in the power transmitting path between an applied state and a released state. More specifically, the invention relates to technology that prevents the direct-coupled clutch from applying if the direct-coupled clutch control solenoid valve fails.
2. Description of the Related Art
A hydraulic pressure control apparatus for a vehicular power transmitting device having a direct-coupled clutch and a continuously variable transmission that includes friction apply devices and a shifting mechanism for continuously (i.e., smoothly) shifting speeds is known which is provided with a first control valve, a second control valve, a direct-coupled clutch control solenoid valve, and a pair of shift control solenoid valves. The first control valve switches the direct-coupled clutch between an applied state and a released state. The second control valve switches the hydraulic pressure supplied to the friction apply devices. The direct-coupled clutch control solenoid valve controls the operation of the first control valve, and the pair of shift control solenoid valves control the operation of the shifting mechanism.
Japanese Patent Application Publication No. JP-A-2003-120797, for example, describes one such hydraulic pressure control apparatus for an automatic transmission (i.e., power transmitting device). The hydraulic pressure control apparatus described in Japanese Patent Application Publication No. JP-A-2003-120797 is for a vehicular continuously variable automatic transmission which includes a torque converter (i.e., a fluid power transmitting device) having a lockup clutch as a direct-coupled clutch and a continuously variable transmission having friction apply devices for switching the direction of rotation between forward and reverse. The described hydraulic pressure control apparatus includes a lockup control valve (a first control valve), a garage shift valve (a second control valve), a first solenoid valve (a direct-coupled clutch control solenoid valve), and a second solenoid valve. The lockup control valve (i.e., the first control valve) switches the lockup clutch between an applied state and a released state. The garage shift valve (i.e., the second control valve) switches the hydraulic pressure supplied to the friction apply devices. The first solenoid valve (i.e., the direct-coupled clutch control solenoid valve) controls the operation of the lockup clutch control valve, and the second solenoid valve controls the operation of the garage shift valve.
If the first solenoid valve fails such that the lockup clutch, i.e., the direct-coupled clutch, is kept applied, the power transmitting path between the engine and the driving wheels will remain mechanically established, which may result in engine stall depending on the state of the vehicle, e.g., when the vehicle is stopped or running at extremely slow speeds. The technology described in Japanese Patent Application Publication No. JP-A-2003-120797 proposes to release the lockup clutch by applying output hydraulic pressure from the second solenoid valve to the lockup clutch control valve if the first solenoid valve fails.
However, a hydraulic pressure control circuit such as the one described in Japanese Patent Application Publication No. JP-A-2003-120797 also includes, in addition to the first and second solenoid valves, third and fourth solenoid valves (i.e., a pair of shift control solenoid valves) for controlling, via a ratio control valve, operation of a shifting mechanism which is formed of a pair of pulleys and a belt that is wound around those pulleys, for continuously, i.e., smoothly, changing speed ratios. Having a plurality of solenoid valves in this way is disadvantageous in terms of both reducing size and costs when there is an increasing need for reductions in these areas.